Heat exchanger with integral phase change material for heating and cooling applications

ABSTRACT

A heat exchanger is provided for use in heating and cooling applications. The heat exchanger includes a heat exchange fluid, at least one heating element for heating the heat exchange fluid, and a plurality of heat exchange pipes positioned in the heat exchanger. The heat exchange pipes are filled with an uncrosslinked high density polyethylene phase change material. The heat exchanger may be used in combination with separate shell and tube type heat exchangers to provide residential heating and hot water heating. The heat exchanger may also be used in combination with an absorption air conditioning unit to provide residential cooling.

BACKGROUND OF THE INVENTION

The present invention relates to a heat exchanger, and more particularly, to a heat exchanger which utilizes phase change materials for the storage and release of thermal energy for use in heating and cooling applications.

Heat exchangers are well known in the art for use in transferring heat from one medium to another, such as from water to air. Heat exchangers are used in a variety of heating and cooling equipment including water heaters and air conditioning units.

One known type of heat exchange unit utilizes a phase change material (PCM). A phase change material is a material that is in the solid phase at low temperatures and in a liquid phase at high temperatures. As a PCM is heated, its temperature increases until it reaches its melting temperature. At its melting temperature, the PCM remains in the solid phase while it absorbs a fixed amount of heat known as the “latent heat of fusion.” Once the PCM absorbs the latent heat of fusion, it changes from a solid to a liquid. The PCM stays in the liquid phase until it releases an amount of heat equal to the latent heat of fusion. As the PCM continues to lose heat, the PCM will change from the liquid phase to the solid phase. As a PCM can store large amounts of heat, the use of PCMs has been desirable for use a variety of applications.

For example, water heaters are known which utilize phase change materials to heat water more effectively. Known phase change materials have a latent heat which is greater than the sensible heat of liquid water. A water heater utilizing a phase change material is described in my U.S. Pat. No. 6,493,507. The heater includes heat exchange tubes with water circulating through the tubes and a phase change material surrounding the tubes such that the heat stored in the phase change material can be transferred through the tubes to the water. The phase change material comprises linear alkyl hydrocarbons formed from synthetic, even-numbered carbon chains. However, a disadvantage of such hydrocarbons is that they tend to sublime, i.e., they transform from the solid state to the gaseous state when heated, without forming a liquid phase. This limits their application in heat and cold storage applications due to the large change in volume which occurs during transformation from a solid to a gas and from a gas to a solid.

Accordingly, there is a need in the art for a heat exchanger utilizing a phase change material to effectively store and release heat for use in residential heating and cooling applications, and which provides improved thermal energy storage and heat transfer.

SUMMARY OF THE INVENTION

Embodiments of the invention meet that need by providing a heat exchanger for the storage and release of heat in heating and cooling applications.

According to one aspect, a heat exchanger for the storage and release of heat is provided which comprises a shell and a plurality of heat exchange pipes positioned in the shell which contain a phase change material therein. The heat exchanger further includes a heat exchange fluid for transferring heat from the phase change material, and at least one inlet and at least one outlet for the transfer of the heat exchange fluid to and from the heat exchanger. The heat exchanger further includes at least one heating element for heating the heat exchange fluid. The heat exchanger preferably includes first and second heating elements positioned adjacent the top and bottom of the heat exchanger.

In one embodiment, the phase change material comprises uncrosslinked high density polyethylene having a density of about 0.96 g/cm³ and a melting temperature of 132° C.

The phase change material may include a number of additives, including from about 1 to 10% by weight carbon black. The phase change material may also include from about 0.05 to 0.5% by weight of a surfactant. In one embodiment, the surfactant comprises a non-ionic surfactant such as polyethylene glycol monolaurate.

The phase change material may further optionally include from about 0.05 to 0.5% by weight zinc stearate which acts as a lubricant.

The heat exchange fluid is preferably selected from the group consisting of ethylene glycol, polypropylene glycol, and glycerin.

In one embodiment, the heat exchanger includes a solenoid valve in conjunction with the inlet or outlet to provide pulsatile flow of the heat exchange fluid and improve heat transfer.

The phase change material is preferably provided in the form of a solid which is molded to fit inside the heat exchange pipes. Typically, both the solid phase change material and the heat exchange pipes will be cylindrical. The heat exchange pipes comprise a metal selected from the group consisting of copper, stainless steel, and glass-coated steel. The heat exchange pipes have an outer diameter of from about 0.5 to about 2.5 inches, and include a closure such as a cap at each end such that said phase change material is sealed therein.

The shell of the heat exchanger preferably has a three-dimensional rectangular or cubic configuration with generally flat sides. Preferably, the shell is comprised of the same metal as the heat exchange pipes. A layer of insulation may be included on the exterior surface of the shell. The insulation is vacuum panel insulation having an R value of about 50 to 60 per inch of thickness.

The heat exchanger may be used in a number of heating or cooling applications including a water heater, a heating unit, or a cooling unit such as an absorption air conditioning system. Where the heat exchanger is used for water heating applications, the water heater includes the (primary) heat exchanger and a separate liquid to liquid heat exchanger which includes an inlet and outlet for the transfer of heat exchange fluid to and from the primary heat exchanger and an inlet and outlet for transporting water to and from the liquid to liquid heat exchanger.

Where the heat exchanger is used in an absorption air conditioning system, an absorption air conditioning unit is provided which receives heat exchange fluid directly from the heat exchanger for transfer to cool air by the air conditioning unit.

Where the heat exchanger is used in a heating system, the system includes the heat exchanger and a separate liquid to air heat exchanger which includes an inlet and outlet for the transfer of heat exchange fluid to and from the primary heat exchanger and an inlet and outlet for receiving and transporting air to and from the liquid to air heat exchanger.

The heat exchanger may also be used in combination with all three applications, i.e., the heat exchanger may be used in combination with a liquid to air heat exchanger for heating air, an absorption air conditioning unit for cooling air, and a liquid to liquid heat exchanger for heating water.

During use of the heat exchanger, the heat exchange pipes are in heat transfer relation to the phase change material contained therein and are in fluid connection with the heat exchange fluid such that the heat exchange fluid heated by the heating element(s) flows around the pipes and heats the phase change material contained therein. The heat is stored in the phase change material and is then transferred through the pipes to the heat exchange fluid flowing at nearly constant temperatures corresponding to the freezing/melting temperature of the phase change material.

Accordingly, it is a feature of embodiments of the invention to provide a heat exchange unit including a plurality of heat exchange pipes having a phase change material therein, and to use of the heat exchange unit in water heating, and in residential heating and cooling applications. These, and other objects and advantages of the present invention, will become apparent from the following drawings, detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partial cross-sectional view of one embodiment of the heat exchanger;

FIG. 1B is a top view of the heat exchanger of FIG. 1;

FIG. 2 is perspective view of a single heat exchange pipe; and

FIG. 3 illustrates another embodiment of the heat exchanger in conjunction with additional heat exchangers; and

FIG. 4 illustrates an enlarged view of the absorption air conditioning unit of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1A and 1B, the heat exchanger 10 of the invention is shown. As shown in FIG. 1A, the heat exchanger 10 comprises a shell 12 including a plurality of heat exchange pipes 16 including a phase change material 18 therein.

The shell 12 is preferably rectangular or cubic in configuration and should be of a sufficient height to be able to accommodate up to about 75 gallons of the heat exchange fluid. The shell 12 may be comprised of copper, stainless steel, or glass-coated steel. It should be appreciated that the shell and the heat exchange pipes should comprise the same material to avoid creating a battery effect due to the chemical reaction between two different metals with the slight acidity of water. For example, if the heat exchange pipes comprise stainless steel, the shell should also be comprised of stainless steel.

As shown, the shell 12 has a flat exterior surface 20 which is surrounded by an insulation material 22. The insulation material 22 preferably covers the entire exposed outside surface 20 of shell 12. Preferably, the insulation material 22 has an “R” value of at least about 50 to 60 per inch. Vacuum panel insulation suitable for use includes vacuum panel insulation available from AccuTemp under the designation ThermoCar®.

The heat exchange fluid is supplied to the heat exchanger 10 during manufacture of the heat exchanger and is filled close to the top of the shell as indicated by fluid level 48 as shown in FIG. 1A. As the system is a closed system, there is no need to add additional heat exchange fluid. When the heat exchanger is used in connection with additional (separate) heat exchangers, the heat exchange fluid is pumped to the heat exchangers and then returned to the (primary) heat exchanger.

It should be appreciated that the heat exchange fluid must have a boiling point which is substantially higher than the 132° C. melting temperature of the uncrosslinked high density polyethylene phase change material. Preferably, the heat exchange fluid has a boiling point of at least 150° C. or higher. Suitable heat exchange fluids for use in the heat exchanger include ethylene glycol, propylene glycol, and glycerin, which are preferably mixed with water in an amount sufficient to maintain the boiling point as desired. A preferred heat exchange fluid is a mixture of ethylene glycol and water in approximately equal amounts.

The heat exchanger includes a number of outlet lines 26, 27 to allow heated heat exchange fluid to flow from the heat exchanger to additional external heat exchangers as will be explained in further detail below. The heat exchanger also includes return lines 72 and 73 to allow return of the heat exchange fluid to the main heat exchanger. If desired, one or more of the inlet or outlet lines may include a programmable solenoid valve 50 as shown on line 26 and/or valve 57 as shown on line 27. The solenoid valve may be partially closed at regular intervals to provide pulsatile flow of the heat exchange fluid to improve heat transfer. The solenoid valve can be programmed to vary both the amplitude and frequency in which the valve is partially closed to provide the desired pressure drop. Variations of frequency from 5 to 60 cycles per minute, and more preferably, from about 15 to 30 cycles per minute are desirable. The valve closure is preferably regulated so as to create a pressure drop of at least 5 psi and preferably up to about 50% of the available pressure.

The heat exchanger further includes heating elements 28 and 30 positioned at the top and bottom portions of the shell. The heating elements 28, 30 preferably comprise resistance heating elements and are connected to a power supply (not shown). To control the temperature of the heat exchange fluid heated by the heating elements, the heat exchanger may also include one or more thermostats (not shown).

The heat exchanger 10 may also include a timer (not shown) connected to the power supply to control the power usage of the heater during designated time periods, e.g. turning off the power supply during peak usage hours.

The heat exchanger further includes a plurality of heat exchange pipes 16 including a phase change material 18 therein. As shown, the heat exchange pipes 16 are positioned vertically in the heat exchange unit. The heat exchange pipes are preferably configured in the shell as shown in the top view of the heat exchanger depicted in FIG. 1B and are preferably held in position by a perforated metal screen (not shown) with circular holes.

Prior to being filled with the phase change material, the pipes are hollow and are preferably comprised of a heat conducting material. Preferably, the pipes are formed from copper, stainless steel, or glass-coated steel. The heat exchange pipes include a cap at each end for sealing the phase change material, which will be described in more detail below. While the pipes and phase change material are shown in cylindrical form, it should be appreciated that both the pipes and phase change material may also vary in shape. For example, the pipes and corresponding phase change material may be square or rectangular in shape.

A preferred phase change material for use in the heat exchanger is an uncrosslinked form of high density polyethylene. The phase change material stores heat energy from the heat exchange fluid and provides heat to the heat exchange fluid when necessary. Phase change materials may be repeatedly converted between solid and liquid phases to utilize their latent heats of fusion to absorb, store and release heat during such phase conversions. These latent heats of fusion are much greater than the sensible heat capacities of water. For example, in phase change materials, the sensible heat (i.e., the amount of heat required to change the temperature of water 1° C.) is 1 cal/g/° C. Thus, over a temperature range of 10° C., water will have a sensible heat of about 10 calories/gram. In contrast, the phase change materials over the same temperature range can store and release between 50 and 60 calories/gram or about 5 times the sensible heat of water. The phase change material also provides useful sensible heat to the system in the amount of about 0.7 calories/gram/° C.

Upon melting and freezing, the phase change material absorbs and releases substantially more energy per unit weight than a sensible heat storage material that is heated or cooled over the same temperature range. The phase change material absorbs and releases a large quantity of latent heat energy in the vicinity of its melting/freezing point. Additionally, the heated exchange fluid is delivered at a nearly constant temperature for applications such as water heating, and home heating or cooling.

A suitable phase change material for use is a 100% uncrosslinked linear crystalline high density polyethylene having a density of 0.96 g/cm³, a latent heat of about 50 calories/gram, and a crystalline melting temperature of about 132° C. The high density polyethylene has a melt index of between about 0.5 and 2.5, and preferably, about 1.0. A preferred phase change material is commercially available from Chevron under the designation Marlex® 60.

The phase change material may further include a number of additives such as carbon black to improve the rate of heat transfer. The phase change material may include from about 1 to 30% by weight carbon black, and more preferably from about 1 to 10% by weight. Preferred for use is an electrically conductive carbon black such as Vulcan® XC72R available from Cabot Corporation.

The phase change material may further include from about 0.05 to 0.5% by weight of a non-ionic surfactant, which aids in obtaining a stable suspension of the carbon black in the phase change material, preventing the carbon black from separating out when the phase change material is in a liquid state. The surfactant preferably comprises polyethylene glycol 200 monolaurate or polyethylene glycol 400 monolaurate, which are commercially available.

The phase change material may further include from about 0.05 to 0.5% by weight zinc stearate, which acts as a lubricant and aids in preventing adhesion of the phase change material to the heat exchange pipes, thus facilitating expansion of the phase change material during melting and contraction during freezing.

In a preferred method of making the phase change material for insertion into the heat exchange pipes, the phase change material, along with any additives, is dry blended to obtain a uniform powdery blend using, for example, a blender. The blend may then be fed into a heated extruder and formed into a cylinder 18 as illustrated in FIG. 2 which will fit into the heat exchange pipes 16. The phase change material is preferably molded into cylinders by an injection molding machine die equipped with multiple cavities for molding cylinders with the desired dimensions.

Referring now to FIG. 2, a single heat exchange pipe 16 for containment of the phase change material 18 is shown which is initially hollow in form and may be formed from metals including, but not limited to, stainless steel, copper, and glass-coated steel. The outer diameter of the pipes may range from about 0.5 to 2.5 inches, more preferably, about 1 to 2 inches, and most preferably about 1.5 inches. The wall thickness of the pipe should be sufficient to withstand normal pressures during operation and is preferably from about 0.030 to 0.125 inches, and more preferably, about 0.060 inches. The pipes 16 are preferably provided in lengths of about 27 inches, while the cylindrical phase change material 18 is about 24 inches in length so as to provide at least about 3 inches of empty space in the pipe once the phase change material has been inserted. This extra space allows for an increase in volume which occurs when the phase change material melts.

Prior to inserting the phase change material 18 in cylindrical form into the pipe, an end cap 40 is applied to one end of the pipe and adhered thereto by soldering, by a high temperature thermosetting adhesive, or by providing mating threads on the pipe and cap.

The open end of the empty pipe is then filled with a source of inert gas such as nitrogen or argon such that most of the oxygen in the pipe is purged. The phase change material 18 is then placed into the pipe and a second end cap 42 is then placed over the open end of the pipe 16 and adhered thereto in a conventional manner as described above. The second end cap 42 includes a hole 44 which has been drilled in the center of the cap which allows residual gas inside the heat exchange pipe to be vented as the phase change material melts and expands for the first time.

After placement of the phase change material into the pipe, the phase change material is then run through a heat cycle, i.e., the phase change material is melted so that it expands in the pipe. When the phase change material has completely melted and has reached its peak volume in the pipe, the hole in the end cap 42 is permanently sealed. The cap may be sealed with a threaded metal screw comprised of the same metal as the pipe, or by the use of a high temperature adhesive or soldering (where copper pipes are used).

The initial heating and expansion of the phase change material should take place prior to final assembly of the water heater, i.e., prior to placing the heat exchange unit inside the shell.

The filled heat exchange pipes are then assembled in a compact configuration consisting of approximately 24 rows with about 24 heat exchange pipes in each row (arranged in a rectangular or square configuration). A perforated metal screen (not shown) having openings to accommodate the pipes may be used at the top and bottom of the pipes to maintain them in proper position. A thin metal strip may also be attached to the bundle of pipes to keep it in place. The bundle of pipes including the phase change material therein is then inserted into the shell of the heat exchanger. By filling the pipes with phase change material in cylindrical form, the phase change material is in direct heat transfer contact with the heat exchange pipes 16 so that, during operation, as the heat exchange fluid surrounds the heat exchange pipes, heat can be transferred from the phase change material 18 to the heat exchange fluid and vice versa.

The thermal energy supplied from the heat exchanger 10 is delivered on a plateau of nearly constant temperature until the latent heat capacity is exhausted. If desired, lower cost off-peak electricity or a green source of energy such as solar photovoltaic or wind driven devices can then be used to supply the energy required to “charge” the phase change material, resulting in significant cost savings for consumers.

Referring again to FIG. 1A, the heat exchanger 10 operates in the following manner. The heating elements 28 and 30 positioned at the top and bottom portions of the shell are preferably controlled by separate thermostats (not shown) to allow heating/melting of the phase change material from the top down. For example, the upper thermostat should be set to a maximum temperature of about 145° C., which is above the 132° C. crystalline melting temperature of the uncrosslinked high density polyethylene phase change material inside the heat exchange pipes. The lower thermostat should be set to a temperature of about 125° C. This will ensure that heating of the phase change material occurs from the top down, so that the phase change material can expand into the empty space at the top of the heat exchange pipes without a build-up of pressure. As the heat exchange fluid is heated, heat from the fluid is transferred from the metal heat exchange pipes 16 to the phase change material 18 contained therein. As the phase change material melts, it will expand into the free space at the top of the pipes.

When the heat exchanger 10 is not in operation, e.g., during peak times of power usage, the phase change material in the heat exchange pipes heats the heat exchange fluid. When the temperature of the heat exchange fluid approaches the freezing/melting point of the phase change material 18, heat is transferred from the phase change material to the heat exchange fluid. As heat is transferred to the heat exchange fluid, the temperature of the fluid is raised. The heated exchange fluid supplied by the heat exchanger 10 is at nearly constant temperature equivalent to the melting temperature of the phase change material 18. This “plateau” of constant temperature remains until the latent heat capacity of the phase material 18 has been used up at which time heat from an external electrical power source is supplied.

Because of the large thermal energy storage capacity of the heat exchanger in comparison with other heat exchange units, the excess heating capacity may be used for additional heating and cooling applications. FIG. 3 illustrates an embodiment in which the heat exchanger is used in conjunction with a separate absorption air conditioning unit 62, a separate liquid to air heat exchanger 64 for providing residential heating, and a separate liquid to liquid heat exchanger 66 for supplying heated water. The primary heat exchanger 10 may be used in conjunction with one or all of these units.

Referring now to FIGS. 3 and 4, absorption air conditioning unit 62 operates similarly to a conventional absorption chiller unit with the exception that it utilizes the heated heat exchange fluid from the primary heat exchanger as a heat source rather than steam or other heat sources. Alternatively, a commercially available unit may be used and modified for use with the primary heat exchanger.

The unit 62 may be adapted to provide cool water or air, depending on the desired end use. The unit preferably utilizes a liquid refrigerant such as lithium bromide/water or ammonia. The unit preferably includes a generator chamber 84 and an evaporator/absorber chamber 86.

In operation, heated heat exchange fluid from the primary heat exchanger enters the unit 62 through line 27 and cycles through the generator 84 such that the lithium bromide/water mixture 75 is heated to boiling. The subsequently cooled heat exchange fluid is then circulated back to the primary heat exchanger through line 72 as shown.

As the lithium bromide-water mixture boils, the water is released as vapor which travels to the top of the generator and is cooled by water circulating through line 90. The vapor then condenses and is carried to the evaporator/absorber chamber 86 via line 88 where the water evaporates and causes cooling of chilled water coils 92. The cooled water can then be directed to various locations to provide cooling as desired or can be directed to a liquid-to-air heat exchanger 94 such that warm air enters through line 68 and exits as cool air via outlet 70. Evaporated water vapor then falls to the bottom of the evaporation/absorption chamber, is cooled by cooling line 96 and condenses at the bottom of the chamber and is sprayed with a mist of lithium bromide-water mixture from line 98. The diluted lithium bromide-water mixture is then circulated back to the generator chamber 84 via line 100. This chilling cycle then repeats and will run continuously as long as heat is supplied from the heated exchange fluid.

Referring again to FIG. 3, heat exchanger 64 is a shell and tube type liquid to air heat exchanger used for heating applications and includes an air inlet 74 and a warm air outlet 76. As ambient air enters through air inlet 74 it is passed over heated pipes containing the heated heat exchange fluid. The heated air is then passed through air outlet 76 for circulation to air ducts in a residential home. The heat exchange fluid then returns to the primary heat exchanger 10 via line 72. Temperature is regulated by controlling the flow of heat exchange fluid from the primary heat exchanger with a pump (not shown) as needed.

Heat exchanger 66 is a shell and tube type liquid to liquid heat exchanger including an inlet 26 for receiving heat exchange fluid from primary heat exchanger 10 and an outlet 73 for transferring heat exchange fluid back to the heat exchanger 10. The heat exchanger 66 further includes a water inlet 80 and a water outlet 78 for transferring heated water to a source such as a faucet. The heat exchanger 66 may further include a temperature control sensor 82 at the water outlet to control the temperature and metering of the hot water. A pump (not shown) controls the flow of the heated fluid from the primary heat exchanger 10 as needed.

Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention. 

1. A heat exchanger for the storage and release of heat comprising: a shell; a plurality of heat exchange pipes positioned in said shell and containing a phase change material therein; a heat exchange fluid for transferring heat from said phase change material; at least one inlet and at least one outlet for the transfer of said heat exchange fluid to and from said heat exchanger; at least one heating element for heating said heat exchange fluid.
 2. The heat exchanger of claim 1 wherein said phase change material comprises uncrosslinked high density polyethylene.
 3. The heat exchanger of claim 2 wherein said high density polyethylene has a density of about 0.96 g/cm³.
 4. The heat exchanger of claim 2 wherein said high density polyethylene has a melting temperature of about 132° C.
 5. The heat exchanger of claim 1 wherein said heat exchange fluid is selected from the group consisting of ethylene glycol, polypropylene glycol, and glycerin.
 6. The heat exchanger of claim 1 wherein said phase change material includes from about 1 to 30% by weight carbon black.
 7. The heat exchanger of claim 1 wherein said phase change material includes from about 0.05 to 0.5% by weight of a non-ionic surfactant.
 8. The heat exchanger of claim 7 wherein said surfactant comprises polyethylene glycol monolaurate.
 9. The heat exchanger of claim 1 wherein said phase change material includes from about 0.05 to 0.5% by weight zinc stearate.
 10. The heat exchanger of claim 1 including first and second heating elements positioned at the top and bottom of said heat exchanger.
 11. The heat exchanger of claim 1 wherein said heat exchange pipes comprise a metal selected from the group consisting of copper, stainless steel, and glass-coated steel.
 12. The heat exchanger of claim 1 wherein said phase change material is in the form of a cylinder.
 13. The heat exchanger of claim 1 wherein each of said heat exchange pipes include a cap at each end such that said phase change material is sealed therein.
 14. The heat exchanger of claim 1 having a three-dimensional rectangular configuration.
 15. The heat exchanger of claim 1 having a cubic configuration.
 16. The heat exchanger of claim 1 including a solenoid valve in conjunction with said inlet or outlet to provide pulsatile flow of said heat exchange fluid.
 17. The heat exchanger of claim 1 further including insulation on the exterior surface of said shell.
 18. The heat exchanger of claim 17 wherein said insulation is vacuum panel insulation having an R value of about 50 to 60 per inch of thickness.
 19. The water heater of claim 1 wherein said heat exchange pipes have an outer diameter of from about 0.5 to about 2.5 inches.
 20. A water heater including the heat exchanger of claim 1 and a separate liquid to liquid heat exchanger including an inlet and outlet for transporting heat exchange fluid and an inlet and outlet for transporting water.
 21. An absorption air conditioning unit including the heat exchanger of claim 1 and an inlet and outlet for transporting heat exchange fluid and an inlet and outlet for transporting air.
 22. A home heating system including the heat exchanger of claim 1 and a separate liquid to air heat exchanger including an inlet and outlet for transporting heat exchange fluid and an inlet and outlet for transporting air.
 23. In combination, the heat exchanger of claim 1 and a) a liquid to liquid heat exchanger for heating water; b) a liquid to air heat exchanger for heating air; and c) an absorption air conditioning unit. 